1. Field of the Invention
The present invention relates to a protection circuit for protecting a switching power supply from heat and the switching power supply.
2. Description of the Related Art
In a conventional protection circuit for protecting a switching power supply from heat, for example, a temperature sensor is provided near a heated part such as a transformer and a diode that is a rectifier element. For example, the temperature sensor outputs voltage that is proportional to detected temperature. When voltage that the temperature sensor outputs becomes voltage of predetermined potential, the protection circuit stops operation of the switching power supply (a first conventional example). For example, as the temperature sensor, a thermistor is used. The thermistor is an element that output voltage increases monotonously when element temperature rises. The protection circuit monitors the output voltage and when the output voltage becomes not less than predetermined potential, it stops operation of the switching power supply. However, in the first conventional example, there is a problem that a power supply becomes OFF when temperature exceeds constant temperature.
Further, for example, in an AV amplifier that amplifies an audio signal, technique that suppresses heat generation by monitoring temperature of a heat sink and by attenuating volume when temperature exceeds constant value is known (a second conventional example). However, in the second conventional example, there is a problem that cannot be handled by control of the amplifier when excessive current flows by failure of a secondary side condenser of the switching power supply.
FIG. 4 is a diagram of a circuit configuration of a protection circuit that is described in JP 4770446 B (a third conventional example). A shottky barrier diode D51B is incorporated in a secondary side rectifier diode D51A. Temperature of the secondary side rectifier diode D51A is detected by leakage current of the shottky barrier diode D51B. Control that reduces output voltage is performed by adding the leakage current to a reference terminal of a shunt regulator Z51. Namely, when temperature of the secondary side rectifier diode D51A rises, output voltage drops and electric power consumption gets smaller. However, in the third conventional example, there is a problem that it is not possible to set threshold at which output voltage begins to change. Further, there is a problem that it is necessary to use a special diode.
In the first to the third conventional example, there are problems mentioned above. A problem that it is not possible to reduce output voltage greatly even if temperature rises is in common among them.